A mysterious figure who used a rare narwhal tusk to help subdue a knife-wielding extremist on London Bridge last month has been identified as a civil servant in Britain’s Justice Ministry. Darryn Frost ended his silence Saturday, telling Britain’s Press Association that he and others reacted instinctively when Usman Khan started stabbing people at a prison rehabilitation program at a hall next to the bridge on Nov. 29. Frost used the rare narwhal tusk to help subdue Khan even though the attacker claimed to be about to detonate a suicide vest, which turned out to be a fake device with no explosives.